This proposal describes a systematic search for gene predisposing to morphine intake and morphine avoidance through the study of inbred mouse strains: known to differ markedly in morphine intake during a two-bottle choice paradigm. When given a choice between a morphine-saccharin solution and a quinine -saccharin solution, the C57BL/6J (B6) mouse will consume greater than 200 mg/kg/day of morphine, showing Straub tail (an opioid- induced tonic contraction of the anal sphincter muscle, in which the tail is held perpendicular to the ground) and 10% mortality. In the same experimental paradigm, the DBA/2J(D2) inbred strain will consume less than 5 mg/kg/day of morphine, with no mortality and no Straub tail. In an attempt to localize genes explaining this strain difference in morphine intake, we have completed an initial quantitative trait loci (QTL) study, in which morphine intake was measured during a two-bottle choice paradigm in 606 F2 mice from a B6 X D2 cross. The 7% highest and 7% lowest morphine-consuming F2 mice (94 mice in total) were genotyped with 150 murine microsatellites, selected to span the autosomal murine genome. Analysis of genotypes and morphine consumption yielded three QTLs on murine chromosomes 10, 6 and 1 which collectively explain 64% of the total variance (85% of the genetic variance) in morphine intake between the B6 and D2 strains. The chromosome 10 locus alone explains 36% of the total variance. DNA samples from this study will be analyzed for new PCR-based markers near the chromosome 10 QTL (defined by the markers D10MIT28 and D10MIT3), to define the region of interest more clearly. Using a modified two-bottle choice paradigm, a second similar QTL study will be conducted to confirm the original QTL results. The chromosome 10 QTL will be studied in a congenic strain. F1 progeny from a D2 X B6 cross will be backcrossed with D2 mice. Those offspring retaining a B6 allele in the D10MIT75-D10MIT3 interval will be backcrossed to D2 and the offspring will be genotyped for these markers. This will be done for 10 generations to create a congenic line of mice. Localization of genes for the complex trait of morphine intake by these methods could be a major advance in understanding opioid addiction.